I'd push them really hard to TRY to find log in. Ask them to check all Google accts etc.
The reason it's important to TRY is that even though you can easily REclaim the listing in a new account, technically it's a violation to have the listing claimed in multiple account AND there's a bigger problem. That other account will still be in partial control of the listing, even after you make the new claim. So if there are violations in that account or the address format is a little off or whatever, it can cause problems wide variety of problem. (Data changing on you, not being able to respond to reviews, violations you can't clean up, and more.)
So you can just claim it, like you would a new listing, even though it's already claimed. But if you end up with problems like too many categories or the data on the live listing keeps flipping on you, then you may need to try to get the other account UNverified.
It's complicated to explain how to do that and varies based on a few factors, so let's just wait and see if you hit problems. If so come back and I'll need to ask you some qs and determine at that time, which route to send you on.
For now try HARD to get access and explain that yes you know how to essentially steal the listing into a new account, it could cause problems down the road, so is really best if they can find the log in for you.
FYI even if you had log in and let's say the customer just moved. You can't just edit and change the address. Need to mark the old location closed and start a new listing at the new address. Crippled I know but that's the only way Google can handle moves right now.
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